Gas lift is a technique where fluid is injected into the string to aid the produced fluids to get to the surface. One way this is done is with side pocket mandrels. Side pocket mandrels are tubular structures fitted into a string at predetermined locations and include an internal side compartment where a valve can be installed without reduction of the string drift dimension. The side pocket has a wall opening and the valve is used to control the rate of fluid that can be injected into the string at the location of each of the side pocket mandrels that are in service for a particular string.
Some designs have tandem valves with separate check valves so that one can be taken out of service without opening communication between the tubing and the casing. Such a design is shown in U.S. Pat. No. 7,228,909 and in model SBRO-DVX side pocket mandrel sold by Weatherford International Ltd. of Houston, Tex. These tandem gas lift valve designs in a side pocket mandrel were built to address issues of capacity or pressure drop in operation and to provide workover capability of removing one of the valve assemblies in a workover and going back in service with a backup. In essence the dual gas lift design of the past ran the gas lift valves in parallel to increase gas injection flow and/or reduce pressure drop across such valves. Check valves associated with each pocket kept tubing pressure in the tubing to protect the surrounding casing from overpressure if the valves are removed from the pockets.
These designs fail to address requirements in many jurisdictions for dual barriers for any wall opening in a tubular string and the surrounding annular space regardless of whether that annular space is open to a formation being produced or is isolated from it with a packer. The present invention offers this capability and a compact design with the possibility of retrofitting of existing side pocket mandrel designs that have two or more locations for inserting valves. An alternative for single valve side pocket mandrels is to run two close to each other and provide control line connection of the valves for capability of running the valves in series. Preferably the passages in the side pocket mandrel can be internally configured to conduct flow in parallel to meet the double barrier requirements of many jurisdictions for isolation of tubular wall openings. Those skilled in the art can get a better understanding of the invention from a review of the description of the preferred embodiment and the associated drawings with an understanding that the full scope of the invention is to be determined by the appended claims.